Income Taxes - Reconciliation of Expected Income Tax Expense at Federal Statutory Rate of 35 Percent to Company's Applicable Income Tax Expense (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 709	$ 719	$ 1,382	$ 1,387
State income tax, at statutory rates, net of federal tax benefit	63	58	114	101
Tax credits and benefits, net of related expenses	(182)	(184)	(346)	(349)
Tax-exempt income	(51)	(51)	(103)	(103)
Noncontrolling interests	(6)	(5)	(10)	(10)
Other items	(5)	10	(30)	17
Applicable income taxes	$ 528	$ 547	$ 1,007	$ 1,043